The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
CO2 capture is an important developmental area for power generation. There are pending regulations for the control of greenhouse gases (GHG) and there are significant activities in developing CO2 capture technologies. One particular area of focus is to combust coal or gas with a rich stream of oxygen. This process, known as oxy-fuel or oxy-firing, results in producing a gas mixture of substantially pure steam and CO2. U.S. Pat. Nos. 5,956,937 and 6,206,684 describe exemplary oxy-fuel combustion power systems in detail. These and all other publications cited are incorporated by reference herein. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
One problematic area for conventional oxy-fuel systems is that the flame temperature, the temperature of combustion products which is in the range of 2,000° F. (1,093° C.) to 5,000° F. (2,760° C.), is too hot for most materials of construction. Thus, there is a need for reducing this flame temperature. Conventional methods use recycled CO2 to reduce the flame temperature, which involves a large recycle loop, requiring large equipment and adding another process for handling CO2. For example, U.S. Pat. No. 4,498,298 utilizes CO2 for cooling purposes. However, CO2 is typically gaseous which limits its ability to absorb the heat of combustion.
To overcome the problems associated with using CO2 as a coolant in combustion systems, water, which is inexpensive and non-toxic, has been utilized as a medium to reduce the flame temperature in several configurations. For example, one such configuration is described in WO2008/097295. In this configuration, a water-borne fuel is combusted with a gaseous fuel in a combustion turbine.
In further known configurations, for example as shown in WO2007/098239, a closed loop oxy-fuel combustion power generation cycle is disclosed. Here, oxygen and a hydrocarbon fuel are combusted to produce a gas mixture of steam and CO2 that drives a turbine directly with the gas mixture. The gas mixture then enters a condenser where CO2 is removed and a high pressure steam is produced that is used as a separate drive gas for a steam turbine. Because gas generators in oxy-fuel combustion power generation systems produce an exceptionally high temperature and pressure drive gas mixture (up to 5,000° F. (2,760° C.) or higher and up to 1,500 psi and higher) due to the utilization of substantially pure oxygen as the oxidizer of the hydrocarbon fuel, the prior art steam and gas turbines do not take full advantage of the high temperature oxy-fuel combustion drive gas.
In yet another configuration, as described in U.S. Pub. No. US2003/0131582, syngas produced within the gasifier is combusted within a gas generator along with oxygen from the air separator, and water is introduced into the gas generator to control the temperature of combustion of the syngas with the oxygen. Products of combustion including steam and CO2 are produced within the gas generator. The combustion products are expanded through a turbine for power output and then separated, such as within a condenser.
Alternatively, U.S. Pat. No. 6,148,602 describes a configuration wherein an oxygen-fired combustor produces drive gas for a turbine. The turbine drives an air compressor and an oxygen compressor on a single shaft. And U.S. Pub. No. US2003/0233830 is directed to a configuration using an oxygen-fired combustor designed for gas turbine operating pressures. The combustor produces a high-temperature gas stream that enters one or more heat exchangers to generate/heat steam, and then enters one or more turbines to generate power. Accordingly, both configurations are limited to turbines.
Thus, while numerous configurations and methods of controlling the flame temperature are known in the art, all (or almost all of them) suffer from one or more disadvantages. One disadvantage is that, as mentioned above, these oxy-fuel configurations are limited to turbines or other expanders driven by the combustion gas mixtures and not other configurations such as boilers. It is well understood that turbines are for specific purposes and can run continuously, whereas boilers are the most commonly used method in thermal power plants. Units operate at close to atmospheric pressure, simplifying the passage of materials through the plant. Thus, the technology represented in the above configurations will not lead itself to adoption for boilers that have different operating conditions.
Another disadvantage with the aforementioned configurations is that water is introduced into the system of interest along with the fuel and oxygen. Thus, because water is not injected independently, it will be difficult to control the flame temperature in the combustion chamber. For example, WO2010/036842 provides a configuration where water/steam is injected into the combustor at various places for controlling the flame temperature. The temperature is controlled in the bulk flue gas rather than in regions such as the boundary walls of the heat exchanger. Thus, this kind of system does not provide any protection for the construction material.
Therefore, there is still a need for oxy-fuel combustion systems in configurations such as boilers and/or for improved configurations and methods of controlling flame temperature in any combustion configuration.